SpaceShipTwo crashed after suffering a mid-flight "anomaly" during a test flight. One of the pilots was killed in the incident, and another was injured. It is not currently known what caused the crash.

Branson has previously said that he would fly on the first public Virgin Galactic trip to the edge of space, along with those of his children who wanted to go.

In an interview in February 2014 Branson said that his private space flight venture could not "afford to lose anyone".

In an interview with Guardian Weekend Magazine, he said: "Everybody who signs up knows this is the birth of a new space programme and understands the risks that go with that.#

"The biggest worry I had was re-entry. NASA has lost about 3 per cent of everyone who's gone into space, and re-entry has been their biggest problem.

"For a government-owned company, you can just about get away with losing 3 per cent of your clients. For a private company you can't really lose anybody."

A spokeswoman for America's Federal Aviation Administration said: "Just after 10am PDT today, ground controllers at the Mojave Spaceport lost contact with SpaceShipTwo, an experimental space flight vehicle. The incident occurred over the Mojave Desert shortly after the space flight vehicle separated from WhiteKnightTwo, the vehicle that carried it aloft.

"Two crew members were on board SpaceShipTwo at the time of the incident. WhiteKnightTwo remained airborne after the incident. The FAA is investigating."

Former Nasa astronaut Michael Massimino said the crash brought back memories of the loss of Space Shuttle Columbia in 2003, which disintegrated during its re-entry into Earth's atmosphere, killing all seven crew members on board.

Mr Massimino told CNN: "It reminds me of when we had our accident in 2003, when we lost Columbia." He added: "It was a really bad day, bad for everyone involved, particularly the families. It's a tragedy, it's a nightmare. When we lost Columbia it was the worst day of my life."